An aircraft tractor is a wheeled prime mover having a tow fork or shovel that engages around or lifts the nose wheel of an aircraft that is to be towed. The aircraft nose wheel can pivot about an upright axis on the aircraft and is normally allowed to pivot freely while it is solidly held by the fork or shovel of the tractor. The tractor is used to move the aircraft, maneuvering it by the captured nose wheel.
A consistent problem with such tractors, as described in U.S. Pat. Nos. 4,745,410 of Schuller, 4,955,777 of Ineson, and 5,013,205 of Schardt as well as in European patent applications 153,378 of Birkeholm and 370,965 of Blanco and in German patent documents 1,953,752 of Lavarenne, 2,150,815 of Xenakis (claiming priority of U.S. Pat. Nos. 804,425 of 13 Oct. 1970) 3,420,004 of Bruggen, 3,439,409 of Lustfeld, 3,534,044, 3,606,363 of Lorenz, 3,808,260 of Henneberg, 3,822,120 of Hengvoss, 4,021,717 of Kutschers, 4,106,079 of Andres, and G 9,111,127, is that the vehicle can tip over if the tractor turns so that it defines an excessively small angle with the aircraft. More particularly, for each aircraft there is a maximum turnout angle that is the maximum angle that the aircraft axis should extend at relative to its direction of movement, which in the case of a towed aircraft corresponds to the longitudinal axis of the towing tractor. If the tractor overturns, that is turns too sharply so that this maximum turnout angle is exceeded, there is a significant danger of damage to the aircraft.
Hence various systems are known to detect the angular position of the aircraft relative to the tractor. In the system of above-cited German 3,534,044 two proximity detectors are positioned on the tractor to either side of its longitudinal axis and both trained on the towed aircraft. The difference between the sensed distances can be translated into the angle of the aircraft relative to the tractor and, when this calculated angle approaches the critical turnout angle, an alarm can be emitted or other action taken. The problem with this system is that the front end of the aircraft is rounded, so accurately determining its position is virtually impossible. Thus such a system is quite inaccurate.
In a simpler and cruder system the flaps that normally cover the port from which the nose wheel emerges are each provided with a red marking that can only be seen by someone in front of the aircraft when the aircraft is angled considerably to the viewing location. The location of the red marks is such that they should only in theory be visible to a driver of a tow tractor when the threshold turnout angle is being approached. Thus when he or she sees these markings, some steering correction should be made.
Neither of these systems is highly accurate or sufficiently automatic to satisfy strict safety standards.